Polarization-maintaining optical fiber
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In fiber optics, a polarization-maintaining optical fiber (PM) is an optical fiber in which the polarization planes of lightwaves launched into the fiber are maintained during propagation with little or no cross-coupling of optical power between the polarization modes. There are several different designs with slightly different characteristics. Most work by inducing stress in the core via a non-circular cladding cross-section, or via rods of another material included within the cladding. The latter kind is sold under different names, depending on the shape of the inserted material generating the stress. Most commonly used are the "Panda" and "Bow-tie" designs. The difference is rather technical, though. The differences in temperature behaviour and the optical properties are negligible.
Polarization-maintaining optical fibers are used in special applications, such as in fiber optic sensing and interferometry. They are also commonly used in telecommunications for the connection between a source laser and a modulator, since the modulator requires polarized light as input. They are rarely used for long-distance transmission, because PM fiber is expensive and has higher attenuation than singlemode fiber